Battery packs, or arrangements of multiple energy storage cells coupled together, are used as power sources in a host of devices. The devices can include all-electric vehicles, hybrid electric vehicles, portable electronic devices, military applications, medical devices, back-up power and distributed energy storage systems in residential and business locations. Improvements in underlying electrochemistry have yielded batteries with improved performance characteristics, such as the Li-ion battery. However, even where multiple energy storage cells are intended to be the same in structure and performance characteristics, there are differences among individual cells. Even with state-of-the-art manufacturing, cells are inherently dissimilar and demonstrate variations in capacity, lifetime, rates of charge/discharge, and other inter-related properties. For example, a battery pack containing a collection of individual cells may exhibit cell-to-cell differences in energy storage capacity of 2-3% when new, and this variation may increase over time (i.e., as the battery pack ages and is charged and discharged multiple times). Since the individual cells of a conventional battery pack may be electrically connected in series to form a series string, the overall performance of the battery pack can be degraded by the performance of the weakest cell in the string. For example, with conventional pack architectures, in a series string of cells, the first cell to become discharged during use may limit the discharge of the other cells in a less than optimum manner.
Conventional approaches have attempted to address the aforementioned problems and improve the performance of battery packs by providing charge balancing, e.g., electronic circuitry intended to equalize cell voltages or states of charge. Such charge-balancing systems include electrical switches and other electrical elements (resistors, capacitors, inductors) present at each cell, or grouping of cells, of the battery pack. In some such systems, resistors may be intermittently connected in parallel with battery cells in a coordinated manner to equalize cell charging voltages by shunting excess current. In other systems, capacitors or inductors are intermittently connected in parallel with cells, such that charge can be transferred from relatively-high-voltage cells to relatively-low-voltage cells. In this manner, performance variations among cells are partially managed, such that the cells of the battery pack converge toward a desired voltage or state of charge.
Conventional switched-resistor, switched-capacitor, and switched-inductor battery management system architectures provide only partial solutions to the problem of performance variation among cells in multi-cell packs. These battery management systems have only a limited ability to accommodate variations in cell capacity, lifetime, maximum rates of charge/discharge, and other properties of multi-cell packs. Moreover, conventional battery management systems, while compensating for usage performance, may actually reduce the usable lifetimes of cells in a battery pack. As a result, in conventional battery packs, useful lifetime is diminished, typically limited by the weakest cells in the pack.